Propylene, an important feedstock in the petrochemical industry, is produced commercially along with ethylene by the thermal cracking of ethane, propane, ethane-propane mixture or naphtha in the presence of steam (ref. L. Kniel et al., in Chemical Industries, Vol. 2 "Ethylene: Key-stone to the Petrochemical Industry, Marcel Dekkor, Inc., New York, 1980 and L. F. Albright et al., eds., in Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983). In the thermal cracking of these paraffins, ethylene is produced in larger quantities than propylene. The cracking process is highly endothermic and hence highly energy intensive and also involves extensive coke formation. Typical results given in the above references indicate that at 93% conversion of propane, 23.7 wt %, 41.4 wt % and 12.9 wt % yield for methane, ethylene and propylene, respectively, could be obtained by the thermal cracking of propane. The ethylene, propylene and also other olefins produced in the cracking process and the unreacted paraffins are separated from the product stream by the well-known cryogenic separation method involving liquefaction and fractionation of hydrocarbons.
The process for the production of propylene and ethylene, based on thermal cracking of propane or other paraffins have following limitations: (1) They are highly endothermic and hence require a large amount of energy for the cracking of paraffins. (2) They involve extensive coke deposition inside the pyrolysis reactor tubes, thus causing increase in pressure drop and hence there are frequent break-downs for removing the coke from the pyrolysis reactor tubes. (3) The life of the pyrolysis reactor tubes is low because of their high temperature operation; the temperature at external surface of the tubes is about 200.degree. C. higher than the temperature inside the tubes.
Catalytic processes based on oxidative dehydrogenation of propane for the productions of propylene area also known in the prior art. A number of catalysts are known for the oxidative dehydrogenation of propane [ref. JP 58,153,538 (matushi Electric Ind. Co. Ltd.), 1983; U.S. Pat. No. 4,472,314, Conner et al., 1984; U.S. Pat. No. 4,547,618 (Mobil Oil Corp.), Porbus and Nancy, 1985; U.S. Pat. No. 4,607,129 (Phillips Petroleum Col.), Lee and Fu, 1985; U.S. Pat. No. 4,886,928, Imai and Schmidt, 1989; U.S. Pat. No. 5,306,858, Salem et al., 1994; JP 08,231,441, Saito et al., 1996 and DE 19,530,454, Baerns et al., 1997]. In the catalytic oxidative propane dehydrogenation processes, the catalyst is deactivated during the process due to the loss of active components from the catalyst by evaporation at hot spots and/or due to the catalyst sintering. The catalysts are also deactivated due to coke deposition on their surface during the process. Moreover, since these processes are highly exothermic, their operation is hazardous.
The present energy crisis and/or high energy cost, and also the environmental pollution problems have created a great need for developing a process for the production of propylene, the demand of which is increasing day-by-day, by non-catalytic oxidative cracking of propane or propane-rich C.sub.2 -C.sub.4 paraffins to propylene and ethylene with high propylene/ethylene mole ratio, which requires little (i.e. much smaller than that required for the thermal cracking process) or no external energy, operates in a most energy efficient manner and with high conversion, selectivity and productivity but without coke formation and also has absolutely no hazards (i.e. very safe operation). This invention is, therefore, made with the following objects so that most of the drawbacks or limitations of the earlier processes could be overcome.